Small scale active heating and cooling devices hold tremendous potential. Potential uses are limited only by the decision as to whether a device, process, or application would benefit from active heating or cooling. Implementation of networked, low-power mesoscopic devices offers obvious advantages compared to traditional active heating and cooling. Practical issues remain in the way of widespread implementation and use of such devices, however. In addition to active heating and cooling devices, e.g., heat pumps, there are additional examples of mesoscale systems that hold promise for a wide range of practical applications. Examples of such mesoscale systems include combustors and evaporators, heat exchangers, and chemical and biological systems.
Mesoscale devices such as these can be defined as ones where the critical physical length scale is on the same order as the governing phenomenological length scale, or ones with critical dimensions that span the microscale to the normal scale (μm<length scale<cm). These large differences in scale pose several challenges in manufacturing. Mesoscopic heat exchangers are needed for a number of applications requiring high heat flux (>1000 W/m2) across thin cross-sections, without incurring excessive pressure losses due to fluid flow in small channels. Enhancement in heat transfer occurs when the effective cross-sectional thickness of a mesoscale heat exchanger matches the thickness over which heat is transferred to the working fluids.
Exemplary potential practical uses of heat exchangers include laptop computer cooling, car seat heating and cooling, airfoil skin heat exchangers, micro-chemical reactors, and compact heat exchangers among others. Another exemplary practical application is the temperature control of clothing. While time is likely to bring the technology to clothing in general, a likely initial application is to chemical and biological warfare protective suits for military personnel operating in extremely hazardous environments. Integrated mesoscopic cooler circuits (IMCC) have been developed by some of the present inventors, and are described, for example in Beebe et al., U.S. Pat. No. 6,148,635, which is incorporated by reference herein. Also see, Shannon, et al., “Integrated Mesoscopic Cooler Circuits (IMCCs).” Proceedings of the ASME, Advanced Energy System Division 39, Symposium on Miniature and Mesoscopic Energy Conversion Devices (1999), p. 75-82.
Others have endeavored to design, fabricate, and mass-produce microchannel (below about 1 mm diameter) heat exchangers for microelectronics cooling and the refrigeration industry. See, P.M. Martin et al, “Microchannel Heat Exchangers for Advanced Climate Control,” Proceedings of the SPIE 2639, (1995), p. 82-88. Delphi Automotive Systems and Modine Manufacturing Company have produced some commercially available mesoscopic heat exchangers made from extruded metals, such as aluminum. Such exchangers are capable of holding high internal pressures and can support large heat fluxes, but typically measure between 0.5 to 1 mm thick, and are not flexible after forming.
Microfabricated thin-film heat exchangers with microchannels 1 mm wide ×30 μm high, made from photosensitive polyimide layers have been reported. Mangriotis, M. D. et al., “Flexible Microfluidic Polyimide Channels,” Transducers 99, The 10th International Conference on Solid-State Sensors and Actuators, Digest of Technical Papers, Sendai, Japan, Jun. 7-10, (1999) p. 772-775. Polyimide was chosen because it is a commercially available high-performance polymer, renowned for its excellent thermal stability, mechanical toughness, high strength, and superior chemical resistance. Fabrication of these heat exchangers utilized batch-mode semiconductor processing of multiple spin-coated layers of DuPont (now HD MicroSystems) PI-2721 polyimide to define specific fluid and vent channel geometries, followed by solvent bonding of a 75 mm thick Kapton HN film to seal the device. See, Glasgow, I. K. et al., “Design Rules for Polyimide Solvent Bonding,” Sensors and Materials 11.5 (1999) p. 269-278.
Even with properly designed vent channel spacing, vapor evolution inherent to the solvent bonding technique can locally degrade the interfacial seal between the microchannels and the Kapton HN film. Thus, large area heat exchangers demonstrated poor structural reliability and thus low fabrication yields. Sealed devices inevitably suffered from very high pressure losses (>100 kPa) over flow lengths of 20 mm, caused by the 30 micron interior channel height. To minimize pressure losses over long flow paths, increased channel heights are required. However, achieving 50 to 150 μm high channels by using multiple spin-coated layers proved to be difficult to scale-up over large planar areas. These examples illustrate some of the difficulties faced in mesoscale device fabrication. Mesoscale devices with vastly different critical dimensions require fabrication methods that can simultaneously meet the tolerances required at both scales.